Having moved up to 600 from a 125. I could not reccomend it enough. Your speed regulation is your right hand, so just use it sparingly if you are worried about it. You decide how fast you go and how quickly you get there, so dont be afraid of a 600. Not sure it's quite the same with a 1000+cc bike, but I find my 600cc much more practical to ride about than the 125 was. Much happier on the motorway, quick squirt to overtake basically anything and helps you appreciate the extra power a bigger bike brings without going crazy.